As is known, such a speed change device comprises a drive pulley and a driven pulley and an endless drive belt. Each pulley comprises a pair of coaxial frustoconical flanges facing each other, namely a movable flange slidably mounted on and fixed for rotation with a shaft and a fixed flange which is not only fixed for rotation with the rotary shaft but also in operation connected axially thereto. Resilient return means urge the movable flange toward the fixed flange.
The present invention is more particularly concerned with such speed change devices in which the resilient return means of each pulley comprise an annular diaphragm spring having a conical or Belleville spring outer peripheral portion which bears axially against the movable flange of the pulley and a central part divided into a plurality of radial fingers by slots alternating therewith and fulcruming on a fulcrum element fixed axially to and for rotation with the shaft of the pulley.
Typically, lugs for coupling the fulcrum element for rotation with the diaphragm spring are provided.
In such present-day devices lugs for rotational coupling are carried by the fulcrum element and they are in engagement in the slots between adjacent pairs of fingers or in passages formed in the fingers themselves for this purpose. In either event they are in engagement with edges formed on the diaphragm spring parallel to the axis of the pulley.
Further, in such present-day devices, the circle on which the diaphragm spring fulcrums on the fulcrum element coincides with the circle on which the lugs for rotational coupling are located between the diaphragm spring and the fulcrum element.
Indeed in such known devices the fulcrum surface for the diaphragm spring on the fulcrum element is a toric surface common to all the radial fingers of the diaphragm spring.
Although such constructions have given satisfaction, they have various drawbacks as will be explained hereinafter.
First of all as the diaphragm spring fulcrums the fulcrum element in line with the lugs coupling the diaphragm spring for rotation with the fulcrum element, the passages through the radial fingers of the diaphragm spring in which the lugs are received inevitably reduce the surface on which the diaphragm spring effectively fulcrums on the fulcrum element.
Now, bearing in mind the relatively high axial load which develops between the diaphragm spring and the fulcrum surface it is important that the axial bearing surface of the diaphragm spring against the fulcrum element is as large as possible in order to minimize the wear at this location.
Furthermore, the radial fingers of the diaphragm spring are bowed and as they fulcrum on a common toric surface of the fulcrum element, the corresponding fulcrum surface for each radial finger is limited and varies depending on the inclination of such a radial finger relative to the axis of the toric fulcrum surface in the course of operation.
Moreover, in known arrangements in which the lugs on the fulcrum element are received in the slots defined between consecutive radial fingers, the width of the slots is caused to change in the course of operation thereby jeopardizing the centering of the diaphragm spring.